1. Field
The present disclosure relates generally to optical fibers and more particularly to multi-clad optical fibers.
2. Description of the Related Art
Optical fiber laser markets have grown significantly in the past few years. Further increase of peak powers will enable a wider range of applications, especially in micro-processing and material processing. However, nonlinear effects limit achievable peak power. A possible key to further peak power increase is a fiber which supports single mode (SM) operation with very large effective area. In previous patents owned by the assignee of the present application, leakage channel fibers (LCFs) were disclosed which can potentially extend effective area by well over an order of magnitude compared to conventional large mode area approaches. Consequently, well over an order of magnitude increase in peak powers from fiber lasers is expected from this new technology.
Many leakage channel fiber designs use low refractive index coating to form a pump guide. This low refractive index coating is sufficient at low pump power levels and can get damaged at high pump powers due to absorption and thermally-induced degradation. With low refractive index coating, a glass part of the fiber is the pump guide. It is preferred in many cases to be able to design the pump guide independent of the fiber dimension. For example, one possible situation is where a large fiber dimension is preferred for resistance to external perturbations such as micro and macro bends, while, a small pump guide is preferred for high pump absorption.
Furthermore, many double clad conventional fibers have used low refractive index polymer to form the pump guide. An annulus region of low refractive index glass has also been used to form the pump guide. Embodiments of such “all-glass” designs may provide an improvement over the designs with low refractive index polymer as these designs may be more resistant to pump induced optical damage. Such designs may not, however, offer sufficient pump mode mixing, which can lead to low pump absorption due to the existence of skew rays.
In some fiber configurations, end caps with no guiding cores have been spliced to fibers to allow mode expansion before the mode reaches the end face of the fiber. This leads to low optical intensity on the end face to minimize surface damage. In the double clad fibers with large cores, a long end cap is required as a consequence of the small diffraction of the large mode. In a counter-directionally pumped amplifier, this long end cap makes coupling pump to the pump guide through the end cap very difficult.